Although computers were once isolated and had minimal or little interaction with other computers, computers now interact with a wide variety of other computers through Local Area Networks (LANs), Wide Area Networks (WANs), dial-up connections, and the like. With the wide-spread growth of the INTERNET®, connectivity between computers has become more important and has opened up many new applications and technologies. The growth of large-scale networks, and the wide-spread availability of low-cost personal computers, has fundamentally changed the way that many people work, interact, communicate, and play.
One increasing popular form of networking may generally be referred to as remote presentation systems, which can use protocols such as Remote Desktop Protocol (RDP) and Independent Computing Architecture (ICA) to share a desktop and other applications executing on a server with a remote client. Such computing systems typically transmit the keyboard presses and mouse clicks or selections from the client to the server, relaying the screen updates back in the other direction over a network connection (e.g., the INTERNET®). As such, the user has the experience as if his or her machine is operating entirely locally, when in reality the client device is only sent screenshots of the desktop or applications as they appear on the server side.
Two common techniques to send graphics data to a client are (1) sending graphic primitives and other operations, which tell a sub-routine on the client side what and how to draw something, and (2) sending a bitmap image to the client for display. Often times, it requires less bandwidth to send a graphics primitive than the visual result of the execution of that graphics primitive. For instance, a text primitive for the 11-character string, “Hello world” could be expressed in 11 Unicode bytes. If each character is to be displayed in a 12-pixel-by-12-pixel area, then the bitmap representation of “Hello world” would require approximately 200-times more bytes to express, and therefore require approximately 200-times more bandwidth to send.
Likewise, the graphics primitive representation of both opaque rectangles, and scrolls of an image require at least an order of magnitude less bandwidth to transfer than their bitmap counterparts. A graphics primitive representation of an opaque rectangle typically requires no more than four bytes. The equivalent bitmap representation may be highly compressed, but will still generally require at least a few kilobytes. Likewise, an image scroll (sometimes referred to as a “screen-to-screen blit”) may be expressed in a graphics primitive as a command that occupies approximately 10 bytes. The bitmap form of moving an 800-pixel-by-400-pixel window may require up to 1.83 MB (the 800-by-400 pixel area, multiplied by 3 bytes-per-pixel and doubled to account for both the old position and the new position), or approximately 10,000 times the space.
There are times when the graphics primitives are not available to send, such as when a previously stored image is to be sent. In such a case, where a bitmap frame is to be sent to a client (such as an application window), the bitmap frame may be subdivided into tiles. Those tiles are then cached on the client side, and when a tile is repeated between two bitmaps, rather than re-sending the client the tile, the server sends an instruction for the client to display the cached tile. This may greatly reduce the bandwidth costs of a remote presentation session, especially where tiles are frequently repeated.
However, even using these techniques for caching bitmap tiles, the bandwidth requirements of sending bitmaps still greatly exceeds that of sending graphics primitives. Further, there are many cases where caching proves to be ineffective, such as when a window is scrolled a number of pixels that is not an even multiple of the number of pixels in a dimension of a tile—even though the image is still substantially the same, because it does not align with the tiles in the same way, already cached tiles are unusable.